Why Does `sizeof()` Return Incorrect Array Sizes When Passed to C Functions?

Understanding the Discrepancy in sizeof() Values for Arrays Passed to Functions in C

In C, arrays can exhibit an unexpected behavior when passed to functions. The question arises: why does the sizeof() operator return an incorrect value when an array is passed to a function, while it provides the correct value before the function call?

To address this, we must understand that arrays in C undergo pointer decay when passed to functions. When an array is passed, it degenerates into a pointer pointing to the array's first element. Consequently, using sizeof() on the function parameter evaluates the size of the pointer instead of the entire array.

A typical example is given in the question:

#include <stdio.h>

void test(int arr[]) {
    int arrSize = (int)(sizeof(arr) / sizeof(arr[0]));
    printf("%d\n", arrSize); // 2 (wrong?!)
}

int main (int argc, const char * argv[]) {
    int point[3] = {50, 30, 12};

    int arrSize = (int)(sizeof(point) / sizeof(point[0]));
    printf("%d\n", arrSize); // 3 (correct :-) )

    test(point);

    return 0;
}

In the main() function, the correct array size is determined by dividing the size of the allocated memory (obtained using sizeof(point)) by the size of each element (sizeof(point[0])). However, in the test() function, the calculated array size is incorrect because sizeof(arr) returns the size of the pointer to the array, not the array itself.

To resolve this issue and allow the function to determine the array's size, it should be passed as a separate parameter:

void test(int arr[], size_t elems) {
   /* ... */
}

int main(int argc, const char * argv[]) {
   int point[3] = {50, 30, 12};
   /* ... */
   test(point, sizeof(point)/sizeof(point[0]));
   /* ... */
}

It's important to note that the sizeof(point)/sizeof(point[0]) trick only works for arrays allocated on the stack. For dynamically allocated arrays, it will return an incorrect result, as the size of the array is not stored with the pointer.

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